These patients are also at risk for warfarin-induced skin necrosis if treated with warfarin and no heparin until warfarin levels are therapeutic; this paradoxical clotting is due to a faster fall in natural anticoagulant proteins than procoagulant proteins in these patients

Heterozygotes have levels 35 - 65% of normal

First thrombotic event occurs between ages 10 - 50 years

Only 30% have thromboembolism, increasing to 75% if coexisting factor V Leiden

Homozygotes (1 per 500 - 750K births) with severely decreased levels present as newborns with DIC and purpura fulminans neonatorum, leading to death unless anticoagulation and replacement therapy with fresh frozen plasma is started

Homozygous protein C deficiency can be cured with liver transplant; however, this is usually too risky so replacement is preferred treatment

Must exclude acquired causes of protein C deficiency

Etiology

Acquired causes of low protein C levels:

Clot formation

Surgery

Liver disease

Warfarin (should be discontinued at least 10 days prior to testing) or Vitamin K antagonist therapy

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